In the Programme for International Student Assessment (PISA), the key knowledge and skills needed in modern society are assessed. So far, however, little is known on the relationship between PISA performance and school achievement. Using Finnish PISA 2015 and 2018 data combined with the national register of education records, the authors studied whether there is correspondence between PISA performance and school achievement. The results showed that the correspondence between PISA performance and school achievement is moderate. The PISA proficiency scores correlated not only with the corresponding grades but also with the grades of other theoretical subjects, indicating that the PISA test assesses a wide range of school achievement. PISA proficiency was related to grades even after controlling for gender and SES.